OWNERS MANUAL FOR WEISS MEDUS D/A CONVERTER
ringing due to the high steepness. In the early days of digital audio these side effects have been
recognized as being one of the main culprits for digital audio to sound bad.
So engineers looked for ways to enhance those filters. They can’t be eliminated because we are
talking laws of physics here. But what if we run the whole thing at higher sampling rates? Like
96kHz or so? With 96kHz we can allow frequencies up to 48kHz, so the reconstruction filter can
have a transition band between 20kHz and 48kHz, a very much relaxed frequency response
indeed. So let’s run the whole at 96kHz or even higher! Well – the CD stays at 44.1kHz. So in
order to have that analog lowpass filter (the reconstruction filter) to run at a relaxed frequency
response we have to change the sampling frequency before the D/A process. Here is where the Up-
sampler comes in. It takes the 44.1kHz from the CD and up-samples it to 88.2kHz or 176.4kHz or
even higher. The output of the up-sampler is then fed to the D/A converters which in turn feeds
the reconstruction filter.
All modern audio D/A converter chips have such an up-sampler (or over-sampler) already built into
the chip. One particular chip, for instance, up-samples the signal by a factor of eight, i.e. 44.1kHz
ends up at 352.8kHz. Such a high sampling frequency relaxes the job of the reconstruction filter
very much, it can be built with a simple 3
rd
order filter.
So, how come that up-samplers are such a big thing in High-End Hi-Fi circles? The problem with
the up-samplers is that they are filters again, digital ones, but still filters. So in essence the
problem of the analog reconstruction filter has been transferred to the digital domain into the up-
sampler filters. The big advantage when doing it in the digital domain is that it can be done with a
linear phase response, which means that there are no strange phase shifts near 20kHz and the
ringing can also be controlled to some extent. Digital filters in turn have other problems and of
course have quite a few degrees of freedom for the designer to specify. This means that the quality
of digital filters can vary at least as much as the quality of analog filters can. So for a High-End Hi-
Fi designer it is a question whether the oversampling filter built into the D/A chips lives up to
his/her expectations. If not, he/she can chose to design his/her own up-sampler and bypass part
of or the whole over-sampler in the D/A chip. This gives the High-End Hi-Fi designer yet another
degree of freedom to optimize the sonic quality of the product.
For the MEDUS we have decided to do part of the up-sampling (the most critical part in fact) in the
Digital Signal Processor (DSP) chip external to the D/A chip.
